1. Field of the Invention
The present invention relates to an auxiliary exposure device for use in an electrophotographic apparatus, and more particularly to a so-called sharp-cut auxiliary exposure device improved, in irradiating end portions of a photosensitive member to form a non-image area thereon, so as to form a clear boundary between the image area and the non-image area.
2. Description of the Prior Art
In an electrophotographic copier or an electrostatic recording apparatus, an electrophotographic image is generally obtained by uniformly charging a photosensitive member for example by corona discharge to provide said member with photosensitivity, then subjecting said photosensitive member to imagewise exposure to form an electrostatic latent image thereon corresponding to an original pattern, and bringing colored charged particles, called toner, close to said photosensitive member to cause selective deposition of said particles onto said electrostatic image thereby obtaining a visible image.
However said uniform charging on the photosensitive member is conventionally conducted over an area wider than the area to be subjected to the imagewise exposure, whereby the end portions of the photosensitive member around the exposed area remain charged because of the absence of exposure and become developed with toner particles resulting in a dirty appearance and in a significiant increease in toner consumption.
Also in an electrophotographic apparatus in which the developed image is transferred onto a transfer material for example a plain paper sheet which is subsequently separated from the photosensitive member, the image area on the photosensitive member is selected narrower than the transfer sheet, and the separation thereof from the photosensitive member is achieved by a separating device such as a separating belt cooperating with the non-image areas located outside said image area. Consequently there may result accumulation of a considerable amount of toner particles in the separating device functioning in contact with the photosensitive member in the areas outside said image area, leading to contamination of said separating device and eventually the obtained copy. Also the toner particles constantly remaining in an elevated amount on the photosensitive member outside the image area gives rise to unnecessary fatigue in the cleaning device used in such apparatus for removing the toner particles remaining on the photosensitive member after the image transfer.
In order to prevent the above-mentioned drawbacks there is already disclosed, for example in the United States Pat. Nos. 3,784,301 and 3,724,940, a method of irradiating the remaining charged area of the photosensitive member with light from an auxiliary light source other than the light source for illuminating the original, thereby avoiding the toner deposition on said area.
FIG. 1 schematically shows a conventional electrophotographic apparatus employing such auxiliary light source, wherein a photosensitive drum 1 receives, through an imaging lens 2, an image of the original in an area 3 of said drum. The effective light beam in this exposure is represented by 4. A transfer sheet 5, after the transfer of the toner image from said drum, is separated from said photosensitive drum by a separating belt 6 positioned therebetween. An area 7 of the drum 1 coming into contact with said separating belt 6 is irradiated by a lamp 8 functioning as the auxiliary light source, of which light emission is defined by a shield 9.
However, in such conventional auxiliary exposure device wherein the light shield 9 is separated from the drum surface an enlarged view being shown in FIG. 2, the boundary portion A between the image area and the non-image area becomes unclear to provide an image with undefined edges since the light from the auxiliary light source 8 overlaps with the original image 4 and further since the stray light from the lamp 8 positioned close to the drum 1 enters the image area through the gap between the shield 9 and said drum 1.